Defects in systemic perfusion (flow of blood) are caused by potentially reversible hemorrhage, sepsis (spread of bacteria) and cardiac arrest. Generally, when a low blood flow is available, the body directs a greater percentage of the flow to the brain and other organs that require continuous blood flow to survive, while decreasing the flow to other organs such as the stomach and intestines which can survive for a longer period without large blood flow. When perfusion to the stomach decreases, carbon dioxide resulting from metabolism is not rapidly carried away and the partial pressure of carbon dioxide increases. The measurement of CO.sub.2 and changes in pH resulting therefrom, are commonly made in the stomach and intestines to determine the extent of perfusion failure in a patient and to determine the effectiveness of treatment.
The measurement of CO.sub.2 is commonly made by threading a catheter through the nasal passage and the esophagus to the stomach, and sometimes through the stomach into the intestines, with a catheter sometimes being threaded through the anus into the colon. These procedures are invasive and can harm the patient. Most measurements of CO.sub.2 are made in the stomach, which still involves more invasion. Furthermore, measurements of CO.sub.2 in the stomach can be affected by CO.sub.2 produced in the stomach by reaction of hydrochloric acid and bicarbonate (produced during the digestion of some foods, especially carbohydrates). A method for obtaining a measurement of CO.sub.2 which was a good indicator of perfusion failure, which was of minimal invasiveness and which was minimally affected by extraneous conditions such as digested fluids in the stomach, would be of considerable value.